Volume range controlling arrangement employing thermionic amplifiers



March 19, 1940. c. E. P. JONES VOLUME RANGE CONTROLLING ARRANGEMENTEMPLOYING THERMIONIC AMPLIFIERS Filed July 28, 1937 2 Sheets-Sheet 1 DEEC. 5/ J var March 19, 1940. c. E. P. JONES 2,193,966

VOLUME RANGE COflTROLLING ARRANGEMENT EMPLOYING THERMIONIC AMPLIFIERSFiled July 2a, 1937 2 Sheets-Sheet 2 g N D m jvmf/vra g I I a (5P. JONESPatented Mar. 19, 1940 UNlTED STATES VOLUME RANGE CONTROLLING ARRANGE-M'ENT EMPLOYING THERMIONIG 'AM- PLIFIERS Cyril Edward Palmer Jones,Cambridge, England Application July 28, 1937, Serial No. 156,066

In Great Britain August 5, 1936 4 Claims.

This invention relates to volume range controlling arrangementsemploying thermionic amplifiers and their use in an electric signaltransmission or recording system as hereinafter re- 6 fered to.

According to one feature of the invention a circuit arrangement forgiving a thermionic amplifying device a desired volume range distortioncharacteristic consists in the inclusion in a negl ative feedback pathcoupling the output of the amplifying device to the input thereof of acircuit or network providing an equal and converse distortioncharacteristic and controlling theattenuation of the circuit or networkin the I'eedback path by a unidirectional current or by'the passagethrough the circuit or network of the signals themselves.

The volume range distortion may consist'in a condensing or expanding ofthe range or a part 20 of the range of volumes of signals impressed onthe amplifying device.

In an electric signal transmission or recording system, for example atelephone system, noises or crosstalk may be introduced into the systemat 25 some point between the transmitter and receiver. Such noise orcrosstalk may be termed unwanted signals. It is desirable that wantedsignals should always be at a greater level than unwanted signals andfor this purpose the wanted sig- 30 nals may be amplified beforetransmission in such a manner that their range of volumes is condensedso that it does not extend below a predetermined level and an excessiveupper limit is avoided. Amplification of this nature has the addedadvantage that it permits the operation of signal operated devices suchas echo suppressors and crosstalk suppressors whilst allowing thedevices to remain inoperative to unwanted signals.

In a system in which the volume range of sig nals is condensed beforetransmission or recording, a volume range restoring device, that is, asuitable expander, is provided at a subsequent point in the system forfaithfulness of reproduction and in two-way systems for stabilityaswell. According to another feature of the invention, one of. these twovolume range distortions (condensation or expansion) is brought about atone point in the system by the inclusion in the normal signaltransmission path of a suitable variable attenuation network associatedwith an amplifying device, while the other volume range distortion(expansion or condensation) is brought about. at another point by anamplifyingdevice with a negative feedback circuit I in which circuit(Cl. Nil-44) is included a network which is a replica of that at thesaid one point, thus ensuring that the volume range of signals leavingthe end of the system is similar to that of the signals entering thebeginning of the system since the two distortions compensate each othersubstantially exactly. The variableattenuation network may include anysuitable device providing a variable impedance or gain, and its controlmay be effected by a unidirectional current or by the passage throughthe network of the signals themselves. The network may, for example,include elements having a non-linear voltage-current characteristic.Unwanted signals arising within the system suffer expansion but nothaving suffered the condensation which the wanted signals havesuiferedit will be seen by'those skilled in the art that it can be arranged thatany unwanted signals are reduced to a level below the lowest levelof thewanted signal, these levels being considered where the signals leave theend of the system.

It is obviously desirable in the proper execution of the invention thatthe networks (whether condensing or expanding) which are in use in twoparts of the system shall be as nearly identical as possible, shalloperate in circuits of like characteristic impedance and shall beprovided with like blessing conditions and control voltage conditionsfor any signal level if either or both of these be necessary. Inpractice, however, the necessity for the exact fulfilment of theseconditions .is not stringent as the law connecting the input level andthe output level of. the condensing network can be made substantiallylinear.

For condensing the range of volumes of signals a condensing network mayinclude an element having a non-linear voltage-current character isticconnected in the input circuit of a thermion ic amplifier and in oneform the network may consist of a pair of rectifiers connected .back toback and shunting the signalling conductors and a resistance in shunttherewith, the impedance of the rectifiers being controlled by aunidirectional voltage obtained by rectifying a portion 45 of thesignals. The network is normallybiassed if necessary to give a lowattenuation. A time delay device is preferably included in the output ofthe rectifier which provides the control voltage. The polarity of therectified voltage applied to the network is such that the shuntingrectifiers offer to signals ashunting impedance which diminishes asthese signals increase in amplitude.

7 Thus, as the input volume rises the network attenuation will risebecause the increase oi rectin fied current is applied to the shuntingrectifiers in their conductive direction. Hence, the range of volumes iscondensed. The rectifiers are conveniently dry plate metal rectifiers.

In the arrangements of the present invention a network substantiallyidentical with that used for condensing the range of volumes is includedin the negative feedback circuit of an amplifier at the point at whichthe received condensed volume range of signals is to be restored to itsoriginal range. The normal gain of the restoring amplifier isconsiderably reduced by feeding back negatively over an attenuationnetwork to the input circuit all or a fraction of the output of signalswhich may appear as a voltage across a resistance in the output circuit.Increase in the attenuation of the network such as may be brought aboutby a loud signal in a manner similar to that described in the precedingparagraph produces a proportional rise of. overall gain of the amplifierthus increasing the range of volumes of signals back to their correctrelationship. The control of the attenuation of the network in thefeedback circuit may be effected in a manner similar to that employedfor the condensing network.

Instead of condensing the range of volumes of the signals in the mannerbefore referred to the condensing may be brought about by theassociation of a thermionic amplifying device with a network adapted forvolume range expansion in the negative feedback circuit, this in effectbringing about the opposite eifect to that brought about by the abovedescribed restoring amplifier. The range of volumes may later beexpanded by the connection in the line circuit of a replica of theexpanding network.

An expanding network may consist of the con- (lensing network alreadydescribed but it should be operated by arranging that it is normallybiased by means for example of a battery so as to have a highattenuation and that the control voltage has a polarity such that theshunt rectifiers have an increasing impedance as signals increase inamplitude. The condensing network described can thus be converted intoan expanding network by providing a substantial bias voltage andconnecting the control voltage obtained from rectifying signal voltagesto oppose this bias, the rectifiers of the network remaining connectedin the same manner and direction. It should be arranged that the controlvoltage due to the maximum signal voltage should not exceed the steadybias voltage. Alternately, an expander network may be formed in otherways. One such way which must be arranged to operate upon signals ofconsiderable amplitude is to transmit from diagonal to diagonal of aWheatstone bridge circuit whose arms taken in order are alternatelyresistors of low temperature coefficient and lamps or other resistors ofhigh temperature coefficient, the signals themselves actually heatingthe resistors forming this bridge and causing the attenuation to fall assignals increase in amplitude. A condensing network may be similarlyformed, the conditions for balance and attenuation being opposite in thetwo cases. The application of a steady biassing voltage to the bridgeelements may be desirable in some cases.

The operation of the network in the feedback circuit of an amplifier maybe interlocked with that of the network in the normal'transmission pathof the signals by the transmission from one to the other of aunidirectional current or an alternating current -'of suitable frequencythe magnitude of which bears a relationship to the instantaneousattenuation of the said one network and which is subsequently rectifiedbefore application to the network in the feedback circuit. In the caseof transmission via a record and reproducing apparatus such interlockingcontrol current must also be recorded if the benefits of interlockingare required. At the said other network the interlocking control currentreplaces a locally derived control current and causes to flow in thesaid other network a current equal to that simultaneously flowing in thesaid one network the interlocking current if necessary being filteredand rectified and with or without amplification to provide the saidequal current in the said other network.

The invention finds use in telephone and like signalling systems for thetransmission of intelligence, music, pictures and the like includingwireless telephone and broadcasting systems and to systems in whichsignals are recorded and stored and the stored record is laterreproduced, where the system is subject to interference by crosstalk ornoise or other disturbing currents.

For the purpose of the present invention it is to be understood that theabove mentioned systems are embraced by the term electric signaltransmission system.

Specific embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings. The drawings arediagrammatic and thermionic amplifiers the circuits of which are notespecially arranged for the purpose of the invention are shown in theconventional manner by triangles the apices oi which point in thedirection of transmission. Although the thermionic valves the circuitsof which are shown are depicted as triodes they may be valves of othersuitable types, e. g., pentodes.

Fig. 1 shows a transmission system in which at its input end, on theleft of the drawing, the range of volumes of signals is condensed bymeans of the network CR associated with the amplifier VI and at theoutput end, on the right hand of the drawing, the volume range isrestored by means of the restoring amplifier RA provided with a negativefeedback circuit FB which includes a volume range condensing network RCRsubstantially identical with that designated CR by which the range ofvolumes of the transmitted signals is condensed. The path between theinput and output ends indicated by light broken lines may be acommunication channel of any kind, wire or wireless, or it may beassumed to include a sound recording device in which the signals arestored. The apparatus on the right would in the latter case be connectedto the sound reproducing device at a time when it is desired toreproduce the record. Fig. 2 shows in skeleton form a modification ofFig. 1 in which the amplifying device with negative feedback circuit isincluded at the input end of the line and in which the network in thefeedback circuit is arranged for volume range expansion and an expandingnetwork and amplifier is connected in the line circuit at a subsequentpoint. Fig. 3 shows another modification in which an interlock betweenthe condensing and restoring amplifier is provided.

For the purpose of example it will be assumed that the system depictedin Fig. 1 is carrying speech and that the light broken lines in thatfigure represent a telephone line. It will be realised that speech mayextend over a volume range from a maximum level to a minimum usefullevel. The maximum may be considered as the greatest volume that islikely to be impressed on the input transformer at the left hand end ofthe drawing and this may be referred to as zero level. The volume ofspeech may fall at some instants to its minimum useful level of, say, 50decibels below zero. I Let it be assumed that a zero level signal istransmitted to the telephone line at zero level and that the line hasits attenuation just compensated by the gain of the receiving apparatuson the right hand side when I mission channels has thus not beenexceeded, al-,

though the weakest signals would be amplified by 25 db. In thearrangement shown in Fig. l the condensing network CR comprises a pairof rectifiers connected back to back shunted across the line and aresistor in shunt therewith and the attenuation of the network iscontrolled by direct unidirectional) current obtained by rectifying theoutput of the thermionic amplifiers vi and Vi! bythe rectifier bridge RBincluded in a rectifying unit RU and fed to midpoint connections of therectifiers and shunting resistor. The network CR and the amplifier Vimay be considered as constituting a volume range condensing amplifier.The network CR has a low attenuation normally, that is in absence of anysignals and might therefore remain unbiased but it is desirable to applya small bias voltage, say. 1 volts, by means of battery B to ensure thatsignal rectification by the shunt rectifiers of CRdoes not take place toany appreciable extent on signals of low amplitude. The rectified outputof the valve V2 is applied to a time delay device comprising a shuntingcondenser K and resistor R which relates the control voltage applied toCB to the average of the speech amplitudes during a short interval oftime. The polarity of the rectified voltage applied to the network OR issuch that the shunting rectifiers offer a diminishing shunting impedanceto speech as its volume rises and so give rising attenuation because theincrease of rectified current is applied to the shunting rectifiers intheir conductive direction. The condenser range of signal volumes isamplified by VI and the amplified speech passes over the line to asubsequent point at which it is amplified by the thermionic amplifierV3. The gains of amplifiers V3 and V i are preferably so adjusted thatthe voltage levels at the'points I, l are equal, but in practice exactequality is not always found essential for correct performance of theinvention. After amplification by V3 the speech passes through theattenuator AN to the restoring amplifying device RA the attenuation ofAN being such as to provide a suitable input level to RA. The device RAconsists of the thermionic valves RVl and RViZ and their connections andhas a negative feedback path FB including a network RCR identical withthe network CR. The connection of the feedback circuit to the inputcircuit of Evil is made over isolating condensers for thepurpose ofexcluding direct current from the i feedback circuit. The output voltageof the valve RV2 developed across the resistance r is fed back over thenetwork RCR whereby a considerable reduction in the gain of the deviceRA is effected. The network RCR comprises the resistor FR and rectifiersM. The resistance of FR is much greater than that of r and representsthe characteristic impedance of the feedback circuit. The combinedresistance of r and rr may be looked upon as the impedance of agenerator and their impedance and the impedance of FR which represents aload on the generator are matched by adjustment of W. This matchedcondition is disturbed by the shunt M across the load circuit to anextent depending on the rectified voltage applied to it, that is on thelevel of the speech actually passing. In a similar manner the impedanceof the resistor in network CR and the impedance of transformer T arematched. The rectifier shunt in CR and that, M, in RCR are thusoperating in circuits of like characteristic impedance; it is alsodesirable that they should operate on signals of like level and theattenuator AN is adjusted to provide an input level to RA such that thesignal level across the rectifiers, that is at the points 2, 2 are equalfor the best results. The bias batteries in RU and RRU should be passingequal small currents in the condition when no signal is passing andequal control currents should pass from RU to CR and from RRU to RCRwhen any signal traverses the system. The attenuation of the network RCRis controlled by a direct current voltage obtained by rectifying theoutput of valves V3 and Vii by means of the rectifier bridge RRB in therectifying unit RRU in the manner described for the networkCR. Theincrease in the attenuation of the network RCR such as may be broughtabout by a loud speech component produces a proportional rise of overallgain of the amplifier thus, increasing the range of volumes back totheir correct relationship. The rectifying unit RRU is substantiallyidentical with the unit RU. It should be noted that even when theattenuation of FIB is greatest there must be an appreciable amount offeedback, enough to reduce the gain of RA by about 15 to 20 db.

Fig. 2 shows in skeleton form a modification 1 the arrangements shown inFig. 1. In Fig. 2 the range of volumes of speech is condensed by meansof an amplifier RA having a volume range expanding network in itsfeedback circuit F13. The apparatus in the two figures bearing likedesignations are similar except for the differences mentioned in thefollowing sentences. For the control of the attenuation of the networkin the feedback circuit a rectifying unit RU connected to the circuit ina similar manner to the similarly designated unit in Fig. l is employedbut the voltage of the bias battery is increased to a value greater thanthe maximum rectified signal voltage to be expected. Since, however, thenetwork in the feedback circuit is for volume range expanding and notcondensing the rectifici's in the rectifier bridge similar to RB will bereversed so that increase in the output of the amplifier in the unitconsequent on receipt of loud speech components will bring aboutdecreasing attenuation of the network and consequent reduction in gainof the amplifier RA. At the subsequent point CR is a similar network tothat of Fig. l but again the bias voltage applied to it iscorrespondingly increased and again the'rectifiers in the rectifierbridge corresponding to RB are reversed so that the effect of increasingapplied rectified voltage to the network is to decrease the attenuationof the network for loud signals and so to expand the volume range of thesignals which are afterwards amplified by the amplifier V5. The voltagelevels at the points corresponding to I, l in Fig. l and at the pointsof connection of the expander network in PB and CR respectively are madeequal by suitable adjustment of the gains of the amplifiers.

Fig. 3 shows in diagrammatic form another modification of thearrangements shown in Fig. 1. In Fig. 3 the amplifier V4 and therectifying unit RRU are not required the control current being providedby RU at the transmitting end via the line conductors. The same currentas circulates in CR due to the bias battery and control voltage could bemade to flow over the line conductors and through the network RCR butleakage due to defective line insulation would be detrimental to thisarrangement. It is therefore preferred that a second rectifying unitRU2, similar to RU, be also connected to amplifier V2, the directcurrent output of RU2 being greater than but proportional to that of RU.The output of RU2 is connected to the line conductors by the well-knownarrangement using a transformer with a split secondary winding, thecircuit being'terminated at the receiving end by a potentiometer P. Asufficient voltage is tapped off this potentiometer to provide for RCR acontrol voltage equal to that provided for CR by RU. The bias battery isomitted in RU2 and a bias battery BB is included in series between P andRCR. The capacity of the condenser K2 in RU2 corresponding to K in RU isless than that of K by an amount necessary to allow for the linecapacity and for that of the smoothing circuit S if this latter is foundto be necessary. Similarly the resistance of the resistor R2 differsfrom that of a R by an amount to take account of the line leakage and ofthe shunt circuit including the line and the potentiometer P. Thusmodified, K2 and R2 with the line-and-P circuit will have the same timeconstant as K and R. Obviously in a recording system a suitableallowance in the values accorded to the components K2, R?! can be madein similar manner but will not include conditions beyond the record.

The smoothing circuit S should not be required under perfect balanceconditions but may be found necessary in practice in spite of theapplication of the control voltage to the network RCR by a midpointconnection.

In an alternative arrangement that may be employed the rectified outputof RU, Fig. 3, may, besides effecting control of the attenuation of thenetwork CR also take effect on an amplifier fed with alternating currentto release the alternating current for transmission over the line to thedistant end where it is rectified to provide the control voltage for thenetwork thereat. The amplifier may be so biased that no alternatingcurrent flows over the line while the circuit is in a quiescentcondition. Variations in the output level of the rectifier unit RU whichare dependent on signal level effect variations in the bias of theamplifier for the control of the amplitude of the alternating current tobe transmitted over the line. The alternating control current is of afrequency outside the range of frequencies of the signals to betransmitted to prevent interference with the signals. To prevent anyundesired effect at the distant end filters may be inserted in the linecircuit between the tapping to the rectifier unit and the volumerestoring amplifier,

the filter being designed to block the transmission of currents of thecontrol frequency.

It should be noted in connection with the figures that the earthconnection shown is the negative connection to the plate battery and itis not necessarily connected to earth, although this is often the case.Where the line circuit arrangement of Fig. 3 is employed it may be ofadvantage to insulate the negative end of the plate battery from earth.

What I claim is:

1. In an electric signal transmission system a transmission circuit andat one end thereof means for condensing the range of volumes of signalsto be transmitted over the circuit including a variable attenuationnetwork, an amplifier and control means for deriving a control currentthe amplitude of which is dependent on the volumes of signals receivedat said end and for applying said control current to said network and tothe transmission circuit for transmission thereover and at the other endof the transmission circuit further means for restoring the volumes ofsignals to their original ranges comprising a thermionic amplifier, aninput circuit thereto, an output circuit therefrom, a feedback circuitconnecting said output circuit to said input circuit and connected inthe feedback circuit a variable attenuation network which is a replicaof that at said first mentioned end of the transmission circuit andmeans for varying the attenuation of the network in the feedback circuitin accordance with the amplitude of the control current transmitted overthe transmission circuit.

2. In an electric signal transmission system a transmission circuit andat one end thereof means for condensing the range of volumes of signalsto be transmitted over the circuit said means including a variableattenuation network and a thermionic amplifier an isolating deviceimpervious to direct current between said means and the transmissioncircuit, a first control means for deriving a direct current theamplitude of which is dependent on the volumes of signals received atsaid end and applying it to said network to vary the attenuationthereof, a second control means for deriving a corresponding directcurrent and applying it to the transmission circuit for transmissionthereover and at the other end of the transmission circuit, furthermeans for restoring the volumes of signals to their original range, anisolating device impervious to direct current connected between thetransmission circuit and said further means, said further meanscomprising a thermionic amplifier, an input circuit thereto, an outputcircuit therefrom, a feedback circuit connecting said output circuit tosaid input circuit and connected in said feedback circuit a variableattenuation network which is a replica of that at said first mentionedend of the transmission circuit and a direct current path connecting thetransmission circuit to the attenuation network in the feedback circuitover which direct current transmitted over the transmission circuit isapplied to the network in the feedback circuit for varying theattenuation thereof.

3. In an electric signal transmission system a transmission circuit andat one end thereof means for condensing the range of volumes of signalsto be transmitted over the circuit including a variable attenuationnetwork, an amplifier and control means for deriving a control currentthe amplitude of which is dependent on the volumes of signals receivedat said end and for applying said control current to said network and tothe transmission circuit for transmission thereover and at the other endof the transmission circuit further means for restoring the volumes ofsignals to their original ranges com prising a thermionic amplifier, aninput circuit thereto, an output circuit therefrom, a negative feedbackcircuit connecting said output circuit to said input circuit andconnected in the feedback circuit a variable attenuation network whichis a replica of that at said first mentioned end of the transmissioncircuit and means for varying the attenuation of the network in thefeedback circuit in accordance with the amplitude of the control currenttransmitted over the transmis sion circuit.

4. In an electric signal transmission system a transmission circuit andat one end thereof means for condensing the range of volumes of signalsto be transmitted over the circuit said means including a variableattenuation network and a thermionic amplifier an isolating deviceimpervious to direct current between said means and the transmissioncircuit, a first control means for deriving a unidirectional current theamplitude of which is dependent on the volumes of signals received atsaid end and applying it to said network to vary the attenuationthereof, a second control means for deriving a correspondingunidirectional current and applying it to the transmission circuit fortransmission thereover and at the other end of the transmission circuit,further means for restoring the volumes of signals to their originalrange, an isolating device impervious to direct current connectedbetween the transmission circuit and said further means, said furthermeans comprising a thermionic amplifier, an input circuit thereto, anoutput circuit therefrom, a negative feedback circuit connecting in saidfeedback circuit a variable attenuation network which is a replica ofthat at said first mentioned end of the transmission circuit and adirect current path connecting the transmission circuit to theattenuation network in the feedback circuit over which unidirectionalcurrent transmitted over the transmission circuit is applied to thenetwork in the feedback circuit for varying the attenuation thereof.

OYRIL EDWARD PALMER JONES.

